Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
High repetition rate diode-pumped solid-state (DPSS) lasers are widely utilized to perform various material processing tasks such as surface cleaning, surface polishing, cutting, and drilling, among others. These applications involve the interaction of high-intensity pulsed laser light generated by the laser with a material surface. Surface reflectivity provides a mechanism for a portion of the light delivered to the material surface to reflect back to the laser system. Optical feedback between the target (e.g., material surface) and the laser during the laser pulse is well investigated. Various methods for suppressing optical feedback during the laser pulse are implemented in modern laser systems.
In one example, Faraday isolators are employed to selectively block reflected light. However, Faraday isolators are only applicable to polarized laser light. The polarization properties of the reflected light may differ significantly from that of the incident light. In addition, even for polarized laser beams, the effectiveness of employing Faraday isolators to attenuate laser light reflected from a target is limited. Faraday isolators are undesirable in high power lasers because of their relatively low transmission efficiency (e.g., less than 90%) and low damage threshold.
In another example, the surface normal of the target may be oriented at an angle with respect to the incident laser beam to prevent reflected light from reentering the laser system. However, this is not an option for applications that require normal incidence of the laser beam onto the processing surface.
As high power, pulsed laser systems are developed, parasitic optical feedback becomes a limiting factor in system operation. Thus, improved methods and systems for attenuating light reflected from target surfaces to the laser system are desired.